Electroless gold plating liquid

ABSTRACT

There is provided a cyanide-free immersion type electroless gold plating liquid that is low in toxicity, can be used near a neutral ph, and has a good solder adhesion and plating film adhesion. The electroless gold plating liquid contains a cyanide-free water-soluble gold compound and a pyrosulfurous acid compound. The plating liquid may further contain a sulfurous acid compound and an aminocarboxylic acid compound. Pyrosulfurous acid or an alkali metal, alkaline earth metal, ammonium, or another such salt thereof can be used as the pyrosulfurous acid compound.

TECHNICAL FIELD

This invention relates to a plating technique, and particularly relatesto a cyanide-free immersion type electroless gold plating liquid.

BACKGROUND ART

Immersion type electroless gold plating liquids have been used to forman intermediate layer in an effort to improve the solder adhesion ofcircuits, terminals, and so forth in printed wiring boards, and toimprove the adhesion of reductive gold plating and the like. Most of thegold plating liquids used for this purpose contain a toxic cyanidecompound as a gold compound, but concerns for environment and workplacerequire cyanide-free gold plating liquids that do not contain toxicsubstances.

Patent applications that have been filed for cyanide-free substitutionalgold plating liquids include those that make use of gold sulfitecompounds (see, for example, Japanese Patent No. 3,030,113 and JapanesePatent Publication No. 2003-13249), those that make use of a salt ofgold sulfites or chloroaurates (see, for example, Japanese PatentPublication No. H8-291389), and those that make use of gold sulfite,gold chloride, gold thiosulfate, or gold mercaptocarboxylates (see, forexample, Japanese Patent Publication No. H10-317157). The electrolessgold plating liquids discussed in these are cyanide-free and thereforelow in toxicity, and can be used close to neutral. But, a problem istheir inferior solder adhesion and plating film adhesion. “Plating filmadhesion” refers to the adhesion between an immersion type electrolessgold plating film and the substrate and, when an immersion typeelectroless gold plating film is used as an intermediate layer, refersto the adhesion between the layers above and below the film.

DISCLOSURE OF THE INVENTION

In light of the above situation, it is an object of the presentinvention to provide a cyanide-free immersion type electroless goldplating liquid that is low in toxicity, can be used near neutral, andbrings good solder adhesion and plating film adhesion.

As a result of researching what adversely affects the plating filmadhesion and solder adhesion of an immersion type electroless goldplating film, the inventors found that the problem is non-uniformsubstitution with the underlying metal plating film, such as anunderlying nickel film. More specifically, solder adhesion and platingfilm adhesion were poor in the case that non-uniform corrosion markssuch as pitting were seen on an underlying nickel film after a goldplating film had been stripped off, because defects of some kind werealso present in an immersion type electroless gold plating film.Conversely, whenever there were no non-uniform corrosion marks to beseen, the solder adhesion and plating film adhesion were good.

Therefore, the inventors researched bath compositions that would notresult in non-uniform corrosion marks in the underlying nickel filmafter stripping the gold off, and as a result discovered that it iseffective to add a pyrosulfurous acid compound, which enables a goldplating film to have good solder adhesion and plating film adhesion.Many patent applications have been filed for cyanide-free immersion typeelectroless gold plating liquids as mentioned above, but none of themcontains a pyrosulfurous acid compound.

Specifically, the present invention is as follows.

(1) An electroless gold plating liquid, containing a cyanide-free watersoluble gold compound and a pyrosulfurous acid compound.

(2) A electroless gold plating liquid according to (1) above, furthercontaining a sulfurous acid compound.

(3) A electroless gold plating liquid according to (1) or (2) above,further containing an aminocarboxylic acid compound.

(4) A gold plated article, produced using the electroless gold platingliquid according to any of (1) to (3) above.

There are no particular restrictions on the cyanide-free water-solublegold compound used in the plating liquid of the present invention, aslong as it is cyanide-free and water-soluble, but it is characterized bycontaining a pyrosulfurous acid compound as an additive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM micrograph of an underlying nickel plating film afterstripping a gold plating film off in Example 1;

FIG. 2 is an SEM micrograph of an underlying nickel plating film afterstripping a gold plating film off in Example 2;

FIG. 3 is an SEM micrograph of an underlying nickel plating film afterstripping a gold plating film off in Comparative Example 1; and

FIG. 4 is an SEM micrograph of an underlying nickel plating film afterstripping a gold plating film off in Comparative Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The electroless gold plating liquid of the present invention will now bedescribed in detail.

The electroless gold plating liquid of the present invention is preparedby dissolving a cyanide-free water-soluble gold compound and apyrosulfurous acid compound in water. There are no particularrestrictions on the cyanide-free water-soluble gold compound as long asit is a gold compound and cyanide-free, but it is preferable to use goldsulfite, gold thiosulfate, gold thiocyanate, chloroauric acid, or a saltof these. The electroless gold plating liquid of the present inventionpreferably contains these gold compounds in an amount of 0.1 to 100 g/L,and more preferably 0.5 to 20 g/L, as the gold concentration in theplating liquid. Substitution of gold will occur much more slowly if thegold concentration is less than 0.1 g/L, but there will be no furtheradvantage to exceeding 100 g/L.

The pyrosulfurous acid compound can be pyrosulfurous acid or an alkalimetal, alkaline earth metal, ammonium, or other such salt thereof. Thepyrosulfurous acid compound is preferably contained in the platingliquid in an amount of 0.1 to 200 g/L, and more preferably 1 to 100 g/L.The effect of preventing non-uniform corrosion of the underlying nickelwill be weak if the pyrosulfurous acid concentration is less than 0.1g/L, but there will be no further advantage to exceeding 200 g/L.

The electroless gold plating liquid of the present invention preferablycontains a sulfurous acid compound as a stabilizer. Examples of thissulfurous acid compound include sulfurous acid and alkali metal,alkaline earth metal, ammonium, and other such salts thereof. Theconcentration of the sulfurous acid compound in the plating liquid ispreferably from 0.1 to 200 g/L, and more preferably 1 to 100 g/L. Thecompound will have no effect as a stabilizer if the concentration isless than 0.1 g/L, but there will be no further advantage to exceeding200 g/L.

The plating liquid of the present invention may further contain anaminocarboxylic acid compound as a complexing agent. Examples ofaminocarboxylic acid compounds include ethylenediaminetetraacetic acid,hydroxyethylethylenediaminetriacetic acid,dihydroxyethylethylenediaminediacetic acid, propanediaminetetraaceticacid, diethylenetriamine pentaacetic acid,triethylenetetraminehexaacetic acid, glycine, glycylglycine,glycylglycylglycine, dihydroxyethylglycine, iminodiacetic acid,hydroxyethyliminodiacetic acid, nitrilotriacetic acid,nitrilotripropionic acid, and alkali metal, alkaline earth metal,ammonium, and other such salts of these. The concentration of theaminocarboxylic acid compound in the plating liquid is preferably from0.1 to 200 g/L, and more preferably 1 to 100 g/L. The effect as acomplexing agent will be weak if the aminocarboxylic acid compoundconcentration is less than 0.1 g/L, but there will be no furtheradvantage to exceeding 200 g/L.

A phosphoric acid compound may also be added as needed as a pH buffer tothe electroless gold plating liquid of the present invention.

Examples of phosphoric acid compounds include phosphoric acid,pyrophosphoric acid or alkali metal, alkaline earth metal, and ammoniumsalts of these, alkali metal dihydrogenphosphates, alkaline earth metaldihydrogenphosphates, ammonium dihydrogenphosphates, di-alkali metalhydrogenphosphates, di-alkaline earth metal hydrogenphosphates, anddiammonium hydrogenphosphates. The concentration of the phosphoric acidcompound in the plating liquid is preferably from 0.1 to 200 g/L, andmore preferably 1 to 100 g/L.

It is preferable to use one of the above-mentioned compounds as a pHbuffer and adjust the pH of the gold plating liquid of the presentinvention to be between 4 and 10, and more preferable to adjust to be apH of from 5 to 9.

The gold plating liquid of the present invention is preferably used at abath temperature of 10 to 95° C., and more preferably 50 to 85° C.

If the pH or bath temperature of the plating liquid is outside theranges given above, there will be problems that the plating rate isslow, or bath decomposition is more apt to occur.

A gold plating film produced by use of the gold plating liquid of thepresent invention after a printed wiring board has been nickel-plated asan underlay for example, has good solder adhesion and plating filmadhesion because there is no non-uniform substitution with theunderlying nickel plating film. No non-uniform corrosion mark is seen inthe underlying nickel film after the gold plating film has been strippedaway.

EXAMPLES

Preferred embodiments of the present invention will now be describedthrough the following Examples and Comparative Examples.

Examples 1 and 2 and Comparative Examples 1 and 2

Plating liquids of the various compositions shown in Table 1 wereprepared as the immersion type electroless gold plating liquids. Thematerial to be plated was a copper-clad printed wiring board with aresist opening diameter of 0.4 mmφ, and plating was performed by thefollowing process.

-   -   acidic degreasing (45° C., 5 min)    -   →soft etching (25° C., 2 min)    -   →acid washing (25° C., 1 min)    -   →activator (KG-522, made by Nikko Metal Plating)        -   (25° C., pH <1.0, 5 min)    -   →acid washing (25° C., 1 min)    -   →electroless nickel plating        -   (plating liquid: KG-530, made by Nikko Metal Plating) (88°            C., pH 4.5, 30 min)    -   →immersion type electroless gold plating (using plating liquid        and plating conditions listed in Table 1)    -   →reductive electroless gold plating        -   (plating liquid: KG-560, made by Nikko Metal Plating) (70°            C., pH 5.0, 30 min)    -   (A water rinsing step lasting 1 minute is inserted between all        steps except acid washing→activator.)

The plated articles thus obtained were evaluated as follows. The stateof corrosion of the underlying nickel plating film was observed at2000-times magnification by SEM after the immersion type electrolessgold plating film had been stripped off with Aurum Stripper 710 (25° C.,0.5 min), a gold stripper made by Nikko Metal Plating, and a check wasperformed by visual observation for corrosion marks (pitting). FIGS. 1to 4 show SEM micrographs of the underlying nickel films after the goldplating films had been stripped away in Examples 1 and 2 and ComparativeExamples 1 and 2, respectively. No pitting or no other non-uniformcorrosion mark was seen in the underlying nickel plating films afterstripping the gold plating films away in Examples 1 and 2, but pittingwas observed in the underlying nickel plating films after stripping thegold plating films away in Comparative Examples 1 and 2.

Solder adhesive strength was measured as follows: the immersion typeelectroless gold plating had been performed; an Sn-37Pb solder ball witha diameter of 0.4 mm φ was placed on it, heated and bonded at a peaktemperature of 240° C. in a reflow oven; then, the strength was measuredwith a series 4000 bond tester made by Deiji.

Plating film adhesion was evaluated as follows: the immersion typeelectroless gold plating was finished; the reductive electroless goldplating was performed; the plating was subjected to a tape peel test,and the plating was visually observed to check if any film had peeledoff. This peel test involved applying a cellophane tape (Cellotape™ madeby Nichiban) to the plating film, then peeling off the tape and visuallychecking to see if the plating film stuck to the tape.

The plating film thickness was measured with an SFT-3200 fluorescentX-ray film thickness gauge made by Seiko Denshi Kogyo.

The evaluation results are given in Table 1.

TABLE 1 Example Comparative Example 1 2 1 2 Bath Gold compound sodiumgold sodium sodium gold potassium gold composition sulfite: 1 g/Lchloroaurate: sulfite: 1 g/L cyanide: 2 g/L (gold) 1 g/L (gold) (gold)(gold) Additive sodium sodium — — pyrosulfite: pyrosulfite: 5 g/L 10 g/LStabilizer sodium sulfite: sodium sulfite: sodium sulfite: 5 g/L citricacid: 5 g/L 10 g/L 30 g/L Complexing ethylenediamine nitriloethylenediamine ethylenediamine agent tetraacetic triacetic acid:tetraacetic tetraacetic acid: 10 g/L 10 g/L acid: 10 g/L acid: 5 g/L pHbuffer sodium sodium sodium — dihydrogen- dihydrogen- dihydrogen-phosphate: 30 g/L phosphate: 30 g/L phosphate: 30 g/L Plating pH 7.5 7.57.5 5.0 conditions Plating temp. (° C.) 80 80 80 90 Plating time (min)10 10 10 5 Evaluation Film thickness 0.05 0.05 0.05 0.05 results (μm)Pitting no no yes yes Solder adhesive 1412 1395 1046 1014 strengthPlating film no peeling no peeling peeled peeled adhesion Solderadhesive strength units: gf (n = 20)

The present invention provides a cyanide-free immersion type electrolessgold plating liquid that is low in toxicity, can be used near neutral,and brings good solder adhesion and plating film adhesion.

1. In a substitutional electroless gold plating liquid for plating goldon a substrate by substitution with a metal in the substrate, theimprovement comprising said gold plating liquid consisting essentiallyof a cyanide free water-soluble gold compound and a pyrosulfurous acidcompound and, optionally, at least one of a sulfurous acid compound as astabilizer and an aminocarboxylic acid compound as a complexing agent.2. The substitutional electroless gold plating liquid of claim 1,further containing a sulfurous acid compound.
 3. The substitutionalelectroless gold plating liquid of claim 1, further containing anaminocarboxylic acid compound.
 4. The substitutional electroless goldplating liquid of claim 1, further containing a sulfurous acid compoundand an aminocarboxylic acid compound.
 5. The substitutional electrolessgold plating liquid of claim 1, wherein the water-soluble gold compoundis present in an amount of 0.5-20 g/L, as the gold concentration in theplating liquid.